1. Field of the Invention
The present invention relates to an autofocus system, and in particular to an autofocus system which automatically moves the position (region) of interest for autofocus (AF) within the field of view of a camera to track a desired object, allowing the system to focus on the object.
2. Related Art
An imaging system like a television camera in which subject images are converted to electrical signals (image signals) through an imaging element (such as CCD) generally employs a contrast-detect method as an autofocus (hereinafter referred to as “AF”) system. The contrast-detect method detects the contrast of a subject image from the image signals captured by the imaging element, controls the focus of the taking lens such that the contrast is highest, and thereby automatically achieves the best focus for the subject.
In an AF system such as a contrast-detect AF method, it is a common practice to process only the subject within part of the field of view of the camera, rather than processing the entire subject within the field of view. A contrast-detect AF system limits the region of interest for AF to part of the subject by extracting image signals of the region of interest for AF from image signals captured through the entire imaging area of the imaging element and controlling the focus such that the contrast of the image signals of the extracted region is highest. As used herein, a subject area to be autofocused within the field of view of the camera, or a subject image area to be autofocused within the image captured by the camera is referred to as an AF area, and a frame indicative of the contour of an AF area is referred to as an AF frame. An example of an AF frame is described in Japanese Patent Application Laid-open No. 2002-365519.
If a single AF area is set at a predetermined fixed position within the field of view, a rectangular AF area is normally positioned in the center of the field of view. It is also known, however, that an operator can specify and change the position of the AF area. As used herein, a point in the field of view (or image to be captured) for determining the position of an AF area in the field of view (or image to be captured) (for example, the position of the center point or any one of the four corner points of a rectangular AF area) is referred to as an AF point. When an operator uses a manipulation unit to specify an AF point in the operation of a television camera and the like, a trackball or a joystick, through which the operator can send commands to move the AF point horizontally and vertically, is used as a manipulation unit.